1. Field of the Invention
This invention relates generally to amplifier circuits, and more particularly to an offset adjustable differential amplifier.
2. Prior Art
The offset voltage of a differential amplifier is one of the errors of that amplifier. The offset error voltage of a differential amplifier can be defined as the output voltage of the amplifier divided by its gain when the inputs of the amplifier are connected together. The offset of a differential amplifier is an imbalanced condition and may be the result of one or more of a variety of factors, such as mismatch of the input differential pair, mismatch of the current loads for the input differential pair, or unequal loading of the input stage, for example. Regardless of the cause of the offset voltage of a differential amplifier, however, its effect will produce a difference in the currents between the input stage and the second stage thereof, which difference will be proportional to the offset error voltage.
Although it may be possible to analyze individual differential amplifiers to determine the specific cause of its offset error voltage and to make a special offset adjusting circuit for each, this approach is not practical for monolithic circuits in which the specific cause of the offset error voltage of each circuit cannot be determined. Furthermore, problems may arise, as will be discussed below, when attempting to employ circuitry which is external to a monolithic circuit for adjusting the offset voltage of that monolithic circuit. Also, once a specific circuit has been designed as a large production monolithic circuit, it is desirable to include as many components of that circuit as possible within the monolithic circuit as internal components, rather than as external components.
Although a differential amplifier may have an offset error voltage, it may be desirable in some applications to adjust that offset voltage to a predetermined level, rather than to completely eliminate it. In the past, the offset voltage of a monolithic differential amplifier has been corrected by employing external circuitry to alter the difference in the currents in the differential amplifier in a direction and by an amount to provide the desired corrected offset voltage. If it is desired to reduce the offset voltage of a differential amplifier to zero, the difference in the currents in that differential amplifier are altered by such additional circuitry in a direction and by an amount sufficient to cancel the effect of the offset voltage. Generally, the temperature coefficient of such additional, external circuitry is different from the temperature coefficient of the internal components of the monolithic amplifier.
The offset error voltage of a differential amplifier is subject to drift with the changes in temperature. Prior known circuitry for adjusting the offset voltage of a differential amplifier is also subject to drift with the changes in temperature. The known prior art circuitry for adjusting the drift in a monolithic differential amplifier produces a drift of the amplifier output with changes in temperature which is substantially greater than the drift which would occur solely as a result of the original, unadjusted offset error voltage of that amplifier without the additional, external offset adjusting circuitry connected thereto. The drift produced as a result of the additional, external circuitry for adjusting the offset voltage results from the fact that the temperature coefficient of the external components is substantially different from the temperature coefficient of the internal components of the monolithic circuit being adjusted. That is, if a gross mismatch of the temperature coefficients of the internal and external components exists, a relatively large drift will result with changes in temperature.
An example of the above mentioned technique for correcting the offset error voltage of a differential amplifier includes the connection of a potentiometer between the differential input pair which can be varied to adjust the difference in currents between the input stage and the second stage thereof by an amount and in a direction to produce the desired amount of adjusted offset voltage. If desired, this potentiometer can be adjusted such that the difference in these currents will be equal to zero, thereby providing a zero offset voltage for the differential amplifier. In a monolithic circuit, such a potentiometer must be external to the amplifier circuit. Generally, the temperature coefficient of such a potentiometer is different than the temperature coefficient of the internal components of the monolithic differential amplifier, thereby resulting in drift of the adjusted offset voltage with changes in temperature.
Another example of offset error voltage adjustment circuitry for a differential amplifier includes an external circuit which is employed for forcing a change in one branch of a current mirror connected to the input differential pair of the amplifier. With changes in temperature, however, the adjusted offset voltage will drift, since the temperature coefficient of the external components is different from the temperature coefficient of the internal components.
Accordingly, it can be appreciated that a need exists for an offset adjustment circuit for a differential amplifier which will produce either no drift or a relatively low drift of the offset adjustment with temperature change.